The present invention relates to an implantable intervertebral connection device, more particularly adapted to secure together two or more vertebrae.
The invention seeks to replace and maintain according to an arrangement as close as possible to normal, the displaced or deficient vertebrae, particularly in the treatment of certain cases of soliosis, cyphosis, exaggerated lordosis, vertebral instability or substantial decrease of the intervertebral space from whatever the cause.
From FR 2 698 533 is known an implantable intervertebral connection device permitting two by two connection of several vertebrae with the aid of pedicular screws prolonged by a cylindrical bearing on which is screwed a spherical ferrule of an inter-screw connection element provided with at its other end either with a spherical ferrule or with a spherical-concave cup coacting with a spherical ferrule of a second connection on a same pedicular screw.
The connection element is of the type with a chain and the securement of the assembly is ensured by nuts engaged on the cylindrical bearing ends and pressed against each ferrule or each ferrule-cup assembly as the case may be, against a spherical seat provided on a hexagonal head of the pedicular screw.
If such a device permits adjustment of the intervertebral spacing, both in extension and retraction, by an action on the chain of each inter-screw connection, and if it also permits taking account of the non-parallelism of the axes of the pedicules in situ by an adjustment of the angle formed by the two connections from the same pedicular screw, thanks to the device with a ball ensuring the junction between the screw and the inter-screw connection elements, this intervertebral connection assembly nevertheless has serious drawbacks relating to its rigidity, preventing the patient particularly from flexion or rotation of the torso.
Moreover, the assembly of the components of the system being of metal such as for example titanium alloy, it is difficult to maintain sufficient gripping of the elements of the connection ball and often a posterior intervention for the emplacement of the intervertebral prosthesis will be necessary to reset the screws for blocking said balls.
The present invention seeks to overcome these various drawbacks by providing an implantable intervertebral connection device, adapted to permit not only adjustment of the intervertebral spacing and an adaptation to the angulation of the pedicules relative to each other, but also giving the patient improved comfort by a design of the device giving it suitable elasticity to permit movements of flexure, rotation of the torso and absorbing shocks.
To this end, the invention has for its object an implantable intervertebral connection device comprising at least two pedicular screws provided in their upper portion with a hexagonal head prolonged by a cylindrical bearing and an inter-screw connection of adjustable length, anchored to each end of said bearings, characterized in that said inter-screw connection is constituted on the one hand by a ball threaded on each bearing and, on the other hand, of two connection elements in prolongation of each other and interconnected by a system of screws for adjusting the spacing, the free ends of the elements being shaped as a concave cup skewered on said bearing and matching said ball such that in line with each pedicular screw the ball will be held in sandwich fashion between two cups of two connections associated with the screw, means being provided to move the two cups toward each other to grip said ball and to secure the assembly of the pedicular screw.
Preferably, the ball is of a resiliently deformable material.
The resilient nature of the rotary junction at the level of each pedicular screw permits absorbing the shocks and better distribution of forces along the prosthesis.
The latter not being constituted by a rigid chain but by a string of elements or links all adapted to have at their ends a certain resilient clearance in all directions, including axially of the pedicular screws, the patient thus has the possibility of moving his torso in flexure and rotation relative to the pelvis, of a certain amplitude, by thus outfitting the spine.
Preferably, the screw system connecting the two elements of an inter-screw connection is constituted by a tapped sleeve secured to one of the elements and a coacting screw-threaded rod, connected to the other element by means of a resilient abutment blockage system.
According to a modified embodiment, the screw system connecting the two elements of an inter-screw connection is constituted by a rod provided at one end with a screw-threaded head engaged in a tapping of a sleeve received in one of the above elements and at its other end, with a cylindrical head received in the other of the elements, means being provided, on the one hand, to permit either the free rotation or the blockage in rotation of said sleeve in the carrying element and, on the other hand, to retain and block in rotation said cylindrical head in the carrying element, a resilient abutment being provided between the head and the element.
Such a device not only permits the adjustment of the intervertebral spacing, not only in extension but also in retraction, but ensures a shock-absorbing connection between the two elements of the connection whilst blocking in rotation the screw portion, thereby guaranteeing the desired spacing.
At the two ends of the intervertebral connection device, the containing-gripping means for the ball of the two end pedicular screws can be lightened, because there is no connection with anything else, by providing the sandwiching of the balls in question between the cup of the single connecting element and a simple cup serving as a washer interposed between the ball and the hexagonal head of the pedicular screw or the blocking nut of the assembly, as the case may be.
Or course the cups have a passage hole for the cylindrical bearing of the pedicular screws of a diameter substantially greater than that of said bearing, so as to impart to the cups a certain omnidirectional angular clearance, but the surfaces, respectively of the hexagonal head and of the blocking nut of the assembly, turned toward the ball, are configured in correspondence with the surface facing the cups.
The ball is preferably spherical, but could if desired have another suitable shape, however the surfaces of the cups turned toward the ball are either spherical or truncated conical or else faceted.